The use of gaseous fuels in internal combustion engines provides a number of advantages over other hydrocarbon fuels, such as conventional diesel. Gaseous fuels such as natural gas may be less expensive than other hydrocarbon fuels, more readily available in remote areas and may burn relatively cleaner during operation. Cleaner burning can result in a reduced amount of combustion byproducts such as carbon monoxide, oxides of nitrogen (NOX), and unburned hydrocarbons. Compression ignited as well as spark ignited gaseous fuel internal combustion engines are well known and widely used.
Compression ignition gaseous fuel internal combustion engines often employ a pilot fuel which is mixed with the primary gaseous fuel and air to form a combustion charge. The pilot fuel can enhance the ability of the combustion charge to be ignited when compressed within a cylinder. Spark ignited gaseous fuel engines employ a spark plug to ignite the combustion charge at a desired time. Engineers have recognized for some time that the inherent clean burning characteristics of certain gaseous fuel engines can be further improved by operating the engines on a relatively lean mixture of fuel and air. Lean burning strategies have been advantageously used in particular to minimize the relative quantity of NOX produced during operation. One lean burning strategy for a gaseous fuel internal combustion engine is disclosed in U.S. Pat. No. 7,007,669 to Willi et al. Willi et al. propose a method for operating an internal combustion engine which includes introducing a pilot fuel into a main combustion chamber, along with air and a primary fuel to form a combustion charge. A fuel injector may be used to inject the pilot fuel into the main combustion chamber, and the combustion charge is ignited in the main combustion chamber via a spark. Ignition of the combustion charge takes place in Willi et al. via a phenomenon known as distributed ignition.